Movement of an automatic transmission of an automobile is controlled by oil which is supplied from a control valve. In a control valve system, a proportional electromagnetic valve is provided to change output pressure of the oil. The proportional electromagnetic valve moves a plunger in accordance with a drive current supplied to the electromagnetic valve, and adjusts passage amount or output pressure of the oil. A conventional proportional electromagnetic valve is described in, for example, Japanese Patent Application Publication No. H11-287351. As described in this publication, in general, a proportional electromagnetic valve is driven by a pulse drive current (PWM signal).
A so-called oil vibration phenomenon that oil is pulsated at an output side of an electromagnetic valve is likely to occur upon the driving of a proportional electromagnetic valve. When oil is supplied to an automatic transmission, it is preferable to suppress the oil vibration. Meanwhile, if a frequency of a PWM signal that is a driving current is increased, the oil vibration may be decreased. In this case, however, durability and reliability of the electromagnetic valve may deteriorate. Meanwhile, when the frequency of the PWM signal that is the driving current is decreased, it is possible to suppress the deterioration in durability and reliability of the electromagnetic valve. However, the oil vibration is increased. As such, the oil vibration and the deterioration in durability and reliability of the electromagnetic valve have a trade-off relationship with each other. Therefore, it is difficult to simultaneously suppress the oil vibration and the durability and reliability of the electromagnetic valve.
A permissible range of oil-vibration and hysteresis is changed by an environmental condition in which the electromagnetic valve is used. For example, in a control valve system, a permissible range of oil pressure and a permissible range of hysteresis are different from each other based on the type of the automatic transmission to be controlled, vehicle type, number of engine revolutions, and other environmental conditions. Therefore, it is preferable to set the frequency of the driving current by estimating the occurrence situation of the oil vibration in a driving period start from the driving of the proportional electromagnetic valve.